The present invention relates to wastewater treatment, and in particular to a high efficiency grit removal system.
Wastewater treatment facilities generally utilize grit handling and removal equipment to isolate and remove coarse solids contained in a waste flow stream prior to the flow stream continuing toward other downstream processes. Utilizing grit removal processes or techniques also aids in reducing maintenance costs and grit related operational difficulties, such as including, but not limited to protecting sludge pumps, piping, centrifuges, and the like from the scouring and wearing action of grit. Grit removal processes also prevent grit from reducing a plant's overall efficiency as a result of clogged sumps and pipes and build-ups in fluid channels, settling basins, flocculation tanks and digestion tanks.
Grit contained in the process flow stream can be removed by mechanical devices, including but not limited to bucket elevators, traveling bridge mechanisms, chains and scrapers, vortex grit tanks, aerated chambers, eductor tube designs and grit pumps. Some products are more effective than others at dealing with the wide range of variables found in the process, such as varying flow stream velocity and quantity.
Wastewater plants may also require that smaller grit sizes, such as 100 mesh or 150 mesh, be removed from the influent waste flow stream, especially with combined systems becoming more prevalent. Wastewater treatment facilities have also cut back on maintenance staffs, therefore the need to reduce maintenance efforts is increasing. Combined systems deliver significantly higher flow at more common intervals and for longer periods of time. Very small grit particles contained in the waste flow streams, such as 100-mesh or 150-mesh, are much more difficult to remove since such particles tend to stay suspended and do not settle out very well unless very large tank liquid volumes are utilized to allow enough detention time within the tank for separation to occur.
In some wastewater treatment facilities, Vortex grit separation systems are utilized as first stage methods. The Vortex grit separation systems can handle larger flows and are run generally on a continuous basis during storm events and routine daily flows. However, these systems are not very efficient and have difficulty removing a high percentage of finer grit particles.
Smaller wastewater treatment facilities, such as those in small towns and municipalities, deal with much smaller water flow streams but also have limited funding. Such facilities need the lowest cost process, yet still have an effective process for reducing abrasive wear to the downstream processes and equipment. One of the efficiency problems with the Vortex grit removal process, which is also found in many smaller facilities, is that it is affected by variations or changes in flow stream conditions. For example, under storm flow conditions, design of the Vortex system tends to loose efficiency rapidly because the design does not adequately handle extreme flow rate variations. Vortex units typically contain a gear driven propeller mechanism to assist in keeping the flow in a desired pattern and to control velocity. However this adds mechanical equipment into a harsh environment, and most equipment located in this type of waste stream will require maintenance. When maintenance must be performed, tanks must be dewatered to allow access to wear-prone equipment located below the water surface. Furthermore, the Vortex process has a relatively low efficiency and has only been marginally effective (approx. 65-85% efficient) at separation and removal of 100 and 65 mesh particles, respectively.
Chain and bucket designs in aerated grit basins can handle 65-mesh requirements, but with all the mechanical components involved, such as including but not limited to buckets, chains, and bearings that are all subject to wear, many cities have gone away from this technology in favor of vortex removal because the vortex removal utilizes less wearable parts. Chain and scraper drag out units have also been utilized as removal devices. Chain and scraper drag out units can handle 65 mesh requirements but generally cannot effectively handle 100 or 150 mesh removal requirements.
Screw conveyors are also used for grit removal. Screw conveyors can act either as a feeder for bucket designs or as the actual removal unit set on an incline to lift and dewater the settled solids. However, inclined screws are inefficient at removal unless run at very slow speeds when handling finer mesh particles such as 100-mesh or 150-mesh, thereby reducing overall capacity. Generally, screw conveyors can remove grit particles heavy enough to settle. However, lighter fine materials tend to remain suspended in the lower bottom end of the tank and when combined with water can change the specific gravity of the pool volume. This suspended fine material then gets easily agitated by the screw conveyor and tends to flow over the effluent weirs, thereby short-circuiting the systems and reducing removal efficiencies.
From a facility design standpoint, it can be difficult to classify grit sizes. Also, the actual quantities of settled grit in the various designs can vary dramatically. Ranges of grit size and quantity can be also compounded by the age of a facility, the condition of the facility and whether the systems are gravity fed or pumped flow. Many plants also have issues with storm flows, runoff water, and dirt infiltration. Typical grit quantities or grit loads can range from 0.8 ft3 to over 500 ft3 per million gallons of flow. A commonly utilized design estimate or guideline is 4-5 ft3 per MGD (millions of gallon per day) for closed sanitary systems and 8-10 ft3 per MGD for combined systems. Additionally, it is important to establish a minimum pool depth along with enough tank volume of the unit in order to allow enough area and detention time for the flow to quies and achieve effective grit settling prior to discharge.
Standard equipment specifications generally require that products guarantee approximately 95% removal of 65-mesh grit or larger with a specific gravity of 2.65. Through numerous studies along with lab and field testing, it has been determined that the pool develops a lower specific gravity as a result of the smaller and lighter grit particles being continually re-suspended by agitation; therefore, an improved method or means to force rapid settling of the finer materials without a prolonged detention time is required and needed in the industry.
Grit particles will typically not settle out when velocities (whether in tanks or fluid channels) are above 0.75 to 1.0 feet per second for 65 mesh or greater size particles. Through lab testing, field testing and site testing, it has been determined that when attempting to remove 100-mesh grit particles, internal tank volume and velocities of approximately 0.25 to 0.5 feet per second should be achieved. Additionally, to remove 150-mesh grit particles, velocities of approximately 0.10 to 0.25 feet per second should be achieved. Table 1 below shows grit particle size data.
TABLE 1Mesh to Micron Conversion Chart.US MESHINCHESMICRONSMILLIMETERS600.00982500.250650.0093* 235*0.235*1000.00591490.1491400.00411050.1051500.0038* 96*0.096*1700.0035 880.088Source: TM Industrial Supply, http://www.fluideng.com/FE/meshmicron.html*extrapolated values
Larger pool areas and lower surface loading rates compensate and allow for increased retention time to promote settling, but dramatically increase capital costs and construction costs. Furthermore, utilization of forced air creates rolling flow patterns within the tanks, thereby also promoting grit settling, but also increasing cost.
The object of a new invention is therefore driven by various needs such that the new design should not rely on large surface areas, large tank volumes, long tanks, and pool depths to allow for effective settling. The design should have the overall ability to separate grit from a waste flow stream quickly and efficiently within a small economical construction footprint. The design should reduce or eliminate moving or rotating mechanical equipment subject to abrasive wear and replacement, including but not limited to screw conveyors, bucket elevators, chain and flight collector or scraper mechanisms, which can typically include chains, sprockets, bearings, flights, wear shoes, and the like. The design should also reduce or eliminate routine maintenance requirements associated with submerged mechanical equipment. The design should simplify basin construction requirements and eliminate complicated construction or circular geometry as required by other designs and processes available on the market today.